Formula & worked example
Qactual=Qrated·(ΔTactual/50)^n; Qrequired=room load·margin.
A 2 kW room load with 10% margin needs 2.2 kW delivered output; lower-temperature systems require a larger ΔT50 catalog rating.
Heating & HVAC engineering calculator · Updated Sep 17, 2026
Radiator Size Calculator — Room Heat Output, BTU/h & ΔT Correction
Convert a room design load into required radiator output and correct a catalog ΔT50 rating to another mean-water-to-room temperature difference.
Inputs
Live result
Required delivered output 2.2 kW (7,506.7116 BTU/h)
Output correction factor 0.51475032
Required catalog rating at ΔT50 4.2739167 kW
Assumptions & limits
- Default radiator exponent is 1.3 and is editable.
- Use manufacturer correction data when available.
Frequently asked questions
What does this Radiator Size Calculator calculate?
Convert a room design load into required radiator output and correct a catalog ΔT50 rating to another mean-water-to-room temperature difference.
What units are supported?
Inputs state their unit basis and results include practical SI and US customary equivalents where useful, so the calculation can be checked against field data and equipment schedules.
Can I use this for final equipment selection?
Use it for transparent first-pass sizing and engineering cross-checks. Final selections should use measured conditions, manufacturer performance data and applicable codes or design standards.